1. Field of the Invention
The present invention relates to a cylindrical product with an inner gear formed by injection molding, a mold for the cylindrical product, and a method of manufacturing the same.
2. Description of the Related Art
A zooming lens barrel is often provided with several types of rotation cylinders for the zooming and focusing operations. In the zooming lens barrel the present applicant has been developing, a rotation cylinder is used to move first and second cylindrical members accommodated therein in an optical axis direction and to transmit rotation to the first and second cylindrical members. The rotation cylinder moves the first cylindrical member back and forth in the optical axis direction, and the rotation is transmitted to the second cylindrical member by an inner gear formed on an inner peripheral surface thereof through gears and the like which move in the optical axis direction together with the first cylindrical member.
In the above-mentioned construction, however, when the inner gear is formed over the whole inner peripheral surface of the rotation cylinder, cam grooves and the like cannot be formed on the same inner peripheral surface. Therefore, a more appropriate shape of a rotation cylinder with a more appropriate shape of an inner gear has been keenly sought; furthermore, a mold for forming the rotation cylinder with such an inner gear from a resin material which can be easily made, and a method of manufacturing the cylinder and mold is desired.
Generally, a metal mold of an injection molding device for a cylindrical object with undercuts, such as an inner gear at the inner periphery thereof, is provided with inner slide cores which are divided into 4 pieces, 6 pieces or more pieces.
However, in case where the undercuts have the shape of an inner gear, the tips of the teeth of the slide cores are liable to be interfered with by the formed inner gear and other divided inner cores; therefore, it is required to shift the timing when adjacent inner cores are driven or to change the velocities that the inner cores are driven when the metal mold is opened. For the above-mentioned purposes, it is necessary to provide a plurality of driving mechanisms and means for controlling the timing and velocity of the drive, resulting in an overall complicated construction of the injection molding machine.
There is a problem about the positions of parting lines of the inner slide core. To increase the thickness of the mold, it is preferable to avoid using the top tooth surface as a parting line and to select the bottom of the teeth to position the parting lines. However, it has been difficult to provide all of the parting lines at the bottom of the mold in relation to the pitch of the inner teeth.
Further, when inner cores are drawn in a radial direction of the inner gear (toward the axis of the inner gear) and the inner gear has, for example, involute teeth, the shoulders of the inner cores are interfered with by the bulged portions of the teeth.